Modern integrated circuits (ICs) are made up of literally millions of active devices, such as diodes and transistors, and passive devices, such as inductors, capacitors and resistors. These devices are initially isolated from each other, but are later interconnected together to form functional circuits. Typical interconnect structures include lateral interconnections, such as metal lines (wirings), and vertical interconnections, such as vias and contacts. Interconnections are increasingly determining the limits of performance and the density of modern ICs. On top of the interconnect structures, contact pads are formed on the interconnect structures and exposed on top surface of the respective chip for IC package. Electrical connections are made through contact pads to connect the chip to a package substrate or another die. Contact pads can be used for wire bonding or flip-chip bonding during IC packaging.
Flip-chip bonding utilizes bumps to establish electrical contact between a chip's contact pads and the package substrate. Structurally, a bump actually contains the bump itself and a so-called under bump metallurgy (UBM) layer located between the bump and a contact pad. An UBM layer generally comprises a diffusion barrier layer (or a glue layer) and a seed layer, arranged in that order, on the contact pad. The bumps themselves, based on the material used, are classified as solder bumps, gold bumps, copper pillar bumps and bumps with mixed metals. Recently, copper pillar bump technology has been proposed. Instead of using a solder bump, the electronic component is connected to a substrate by a copper pillar bump, which achieves finer pitch with minimum probability of bump bridging, reduces the capacitance load for the circuits, and allows the electronic component to perform at higher frequencies.
However, there are challenges to implementing such features and processes in IC fabrication. For example, delamination between an inter-metal dielectric (IMD) layer and an interconnect structure overlying the integrated circuits due to high stress from a copper pillar. Accordingly, what is needed is an improved bump structure and method of bump formation.